An arrangement in which, when battery modules in which a plurality of battery cells are housed in the interior of a rectangular parallelepiped-shaped exterior case are superimposed vertically in two tiers, in order to prevent the exterior case of the lower tier battery module from being crushed by the weight of the upper tier battery module, a plurality of column-shaped strengthening members (spacers) are provided on a side face of the exterior case of the lower tier battery module is known from Patent Document 1 below.